This puzzle is not underground.

You'll be standing on a man-made structure when you see it.

You'll look downward to see it.

Overlooking the beach is a ruin with a set of stairs going up it, and a smaller set of stairs underneath the larger stairs. Walk up the smaller stairs, and walk along, looking at the darker color of the wet sand on the shoreline. This is the path to trace.

-

You'll look down toward the beach below.

Look where the sand meets the water.